AC DRIVE APPLICATION ISSUES
Introduction
The ac input characteristics are
1. Diode source current unbalance
2. AC power factor
3. AC input power change with AC input voltage The ac output characteristics cover
1. IGBT switching transients
2. Cabling details for ac drives
3. Motor, cable, and power system grounding
4. Motor bearing currents
Diode Source Current Unbalance
Any unbalance in the ac source voltage will cause very large ac unbalances to flow in non- regenerative ac drives. This is due to the very low impedance of the dc link energy storage capacitor. In general, the ac current unbalance is about 10 to 20 times larger than the ac volt- age unbalance (depending on whether there is a dc link inductor and what its size is). Since most ac power systems have an unbalance of 1 to 2 percent, these problems can occur:
● The rating of the ac cables must increase.
● The rating of the supply and any ac switchgear must also increase.
● The rating of the diode bridge may also require an increase due to the additional dissipa- tion in the semiconductors. The consequences of the unbalance can be very significant because the voltage unbalance in typical power systems can be 2 to 3 percent. This prob- lem will become more significant if the ac feeder has single-phase loads because these loads will increase the already existing voltage unbalances.